1. Technical Field
The present invention relates to an optical module used for optical communications and an optical communication device.
2. Related Art
Optical modules have various forms, and there is known, as one of the forms, an optical module having a structure for receiving a part of the light output from a light emitting element by a monitoring light receiving element to monitor the intensity of the light. In this case, an intensity variation in the output light caused by ambient temperature may sometimes occur. Therefore, in the past, the temperature characteristic of each of the members such as a monitoring light receiving element or a lens has been set as low as possible to maintain a proportional relationship between the monitor light intensity and the fiber coupling light intensity independently of the ambient temperature (see, e.g., JP-A-2004-72072 and JP-A-10-65189).
Further, JP-A-8-236807 discloses a technology for reducing the variation in the optical output caused by the temperature by using a multilayer transmission film (27) having low transmittance to the emission wavelength providing the light emitted from the light emitting layer (24) with high emission intensity.
The inventors have considered making the emission intensity of the light emitting element constant by performing feedback control of a drive current of the light emitting element in accordance with the emission intensity of the optical module. In particular, in the case of using a vertical cavity surface emitting laser (VCSEL) is used as the light emission element, since a large intensity variation is caused by the ambient temperature, which causes a phenomenon that the higher the temperature rises, the lower the emission intensity becomes, feedback control (auto power control (APC)) for controlling the drive current so that the higher the ambient temperature becomes, the lager drive current is applied becomes important for keeping the intensity constant.
On the other hand, in an optical module in 850 nm band or higher, the drive current needs to be larger in order for operating at high speed even at low temperature. This is because there is caused a phenomenon that the operation band of the element is lowered when the drive current becomes small at low temperature. In this case, since the optical output becomes large, the light intensity is attenuated (attenuation) using a partial reflection film, thereby achieving safely of human eyes.
However, if the feedback control of the drive current of the light emitting element is performed based on the attenuated light intensity, the drive current is further increased at high temperature. As a result, in some cases, the drive current exceeds a limit value (an upper limit value), which causes a problem that a proper operation becomes difficult at a certain temperature or higher because of an insufficient output. On the contrary, when operating it with a constant drive current, although the use temperature range is widened, the intensity variation becomes large, as described above.
Further, regarding a laser beam, there is an amount of current with which the laser beam is emitted most efficiently at each temperature, and if driving with such an amount of current is possible, the use temperature range can be widened, and further the transmission characteristics can be improved (e.g., achieving higher-speed, lower-noise operation).